It is known to use self-lubricating sintered sliding members in an unlubricated machine as bushings. One example of such bushings comprises a copper or stainless steel parent layer into which a solid lubricant such as graphite, molybdenum disulfide or tungsten disulfide is mixed (e.g., SL alloys produced by TOSHIBA TUNGALOY CO., LTD.). An inexpensive self-lubricating sintered sliding member is currently available, which is formed from a special-type plastic material containing a solid lubricant. Previously, the inventors have proposed self-lubricating sintered sliding members capable of sustaining high load in Japanese Patent Applications No. 63-190994 (1988) and No. 2-14380 (1990). The sliding members disclosed in these applications are formed by mixing and dispersing a solid lubricant such as graphite into a parent material made from a sintered iron-family material.
The self-lubricating sintered sliding member made from plastic material, however, has the disadvantage that when the pressure on the sliding surface exceeds 200 kg/cm.sup.2, "permanent set" occurs, resulting in a considerably short wear life. The self-lubricating sintered sliding members of other types also reveal the same problem when the pressure on the sliding surface exceeds 500 kg/cm.sup.2.
There has been proposed a double-layer bushing designed for use in high load applications where the bushing is subjected to an offset load or peak load. This bushing comprises (i) an inner layer having a depth of about 1 mm from the cylindrical inner surface and made from an iron-family, self-lubricating contact material reinforced and hardened by heat treatment and (ii) an outer layer enclosing the inner layer and made from an ordinary, high-strength, iron-family, sintered material. This double-layer bushing suffers from the problem that if it is subjected to an excessive offset load, abnormal wear would occur or micro cracks would be created in such a fashion that the solid lubricant particles dispersing within the parent layer made from the sintered iron-family material or the regions around them are linked to one another by the cracks.
The tissues of the above prior art self-lubricating sintered sliding members were observed after their sliding movement and it was found that the following phenomenons (1) to (5) were associated with the problems encountered by them.
(1) The solid lubricant particles become close to or come in contact with one another because of the segregation of the solid lubricant particles and the flattening of them caused by pressurization during press molding, so that cracking starts from the most brittle parts.
(2) In the case where graphite is mixed into a sintered parent layer made from an iron-family material for example, a brittle carbide such as cementite is created in the vicinity of graphite and in the grain boundary of the sintered parent layer by the reaction during sintering. This weakens the sintered parent layer so that the bushing becomes more susceptible to distraction and abnormal wear.
(3) The occurrence of the phenomenon (1) may be restricted to some extent by increasing the particle size of the solid lubricant particles and the distance between them. However, the lubricant particles are significantly flattened by the pressurization during press molding and the metal particles of the parent layer bite into the layer, causing notch-like configuration, so that the bushing is subjected to stress concentration and, in consequence, distraction.
(4) In the early period of abnormal wear caused for the above reasons, the bushing seems to function without making troubles and abnormal noise, because a large amount of wear chip powder including a solid lubricant is produced. However, the lubricating effect of the wear chip powder decreases from a certain moment, starting abnormal noise generation.
(5) Since a solid lubricant cannot be used inordinately in view of strength, the conformability of the bushing cannot be obtained in the initial stage of operation. Therefore, there is a need for sufficient machine warm-up in order to prevent abnormal noise generation, seizure which would occur at the beginning of operation and the like. Insufficient use of a solid lubricant also causes the adhesion and galling of the bushing relative to the member over which the bushing slides.
One attempt to solve the above problems (1) to (3) which would arise when the parent layer is made from a sintered material is such that holes or grooves are formed in an ingot material by machining and these holes are filled with a solid lubricant. Bushings formed by this method are commercially available (e.g., bushings under the code of S0#50 SP2 produced by SANKYO OILLESS INC.). However, such bushings are costly since a number of holes or grooves made by machining are required and leave the problems (4) and (5) unsolved. In addition, seizure and abnormal wear are likely to occur when the rocking angle of the bushings becomes small, unless the pitch of the solid-lubricant-embedding holes is reduced. To solve this problem requires formation of more embedding holes, resulting in higher cost.
The present invention is directed to overcoming the above problems and therefore one of the objects of the invention is to provide a self-lubricating sintered sliding member and its producing method, the member having the ability of exerting superior resistance to seizure and wear even in offset load and high load applications.